Two-part closures typically comprise a cap or disc which is held onto a container by a skirt, which is sometimes referred to as a ring, or overcap. The two parts may be of the same material, or different materials as in a combination closure or “combo-cap” which uses a metal disc and a plastic skirt. The skirt often defines threads or lugs, which cooperate with complementary features on a container neck. Sealing material is conventionally applied to the disc to form a seal between the disc and the mouth of the container.
There are many examples of two-part closures, of which U.S. Pat. No. 3,446,381 A (PODESTA ET AL) is one of the older examples. That patent describes a two-part metallic cap with a separate disc and “sleeve-like” skirt. Another two-piece metallic cap from the same inventor is shown in U.S. Pat. No. 3,836,033 A (PODESTA). In U.S. Pat. No. 3,836,033 A the disc has an edge portion which projects beyond the container mouth, an annular portion which sits on the top edge of the container mouth and a middle portion covering the opening of the container. The entire middle portion of the disc moves between concave and convex configurations by pivoting of the annular portion about the top edge of the container mouth. Both of these closures can be used for containers such as glass bottles, pots and the like.
It has also been popular to provide screw threads or lugs on the skirt, whether this is of metal or of plastics material. For example, the skirt may be part of a threaded outer cap, as in U.S. Pat. No. 4,473,163 B (ERNST & CO.) in which the outer cap prises the inner cap off the container during unscrewing. This is particularly useful when the closure is used for the packaging of food products, which during processing “pull” a negative pressure, often called a “vacuum”. This vacuum creates a substantial resistance to opening. The two-piece cap of EP 1686070 A (PLATO PRODUCT CONSULTANTS V.O.F.) has a special feature to reduce torque on opening. This feature is similar to a dimple, which during opening, pushes up a disc-shaped inner cap to overcome the forces between jar and closure, which create the sealing of the package.
The 2-piece closure design allows the breaking of the container seal to be controlled. The initial twisting of the skirt is sometimes used to activate a tamper-evident feature, such as breaking a tamper-evident band, whilst the disc remains sealed to the container. Thereafter, a further feature may be provided on the skirt to prise the disc from the container, thereby breaking the seal between the disc and the container to equalise any difference between the internal pressure in the container and the external environment. This 2-stage opening reduces the torque required to open the closure and allows the closure to be removed more easily.
Although known features such as these can reduce the force necessary to break vacuum, there are still problems inherent in the combined opening force requirements of unscrewing and breaking the vacuum. This invention seeks to overcome those problems.